1. Field of the Invention:
The present invention relates to a laser generator with phase mode-locking, the generator comprising a resonant cavity delimited by a reflecting rear mirror, and by a semi-reflecting outlet mirror.
2. Description of the Related Art:
FIG. 1 is a diagram showing the disposition in a conventional laser source of a rear mirror 1 and an outlet mirror 2 defining a path between them of length L with the light passing through an active amplifier medium 3 lying on the path. Given the various types of laser source that exist, FIG. 1 is not limiting in the way it indicates the nature and the geometry of the active medium 3.
A conventional laser source constitutes an oscillator, and by virtue of the multiple back-and-forth passes of the light through the resonant cavity of the oscillator, and assuming that no special precautions are taken, it provides laser emission whose time structure is that of a periodic noise of period T=2L/c (1) where L is the distance between the two mirrors 1 and 2, and c is the speed of light. The shape of the light intensity i(t) as a function of time is shown in FIG. 3. FIG. 3 shows the correlation time dt which represents the reciprocal of the spectrum width .DELTA.f of the emitted signal. FIG. 4 shows the frequency spectrum I(f) of the periodic noise i(t) shown in FIG. 3. It can be seen that the frequency spectrum corresponding to conventional laser emission comprises a plurality of groups of equidistant lines with the intervals df between the groups being the reciprocal of the period T of the time signal i(t). Thus, df=c/2L (2) where L is the distance between the two mirrors 1 and 2, and c is the speed of light.
However, in the FIG. 4 frequency spectrum, the intensities and the phases of the groups of equidistant lines are distributed randomly. It is thus something of a misuse of language for the groups of frequencies at which laser energy is concentrated to be called "modes".
In conventional laser emission of the periodic noise type, the amplitude of the power fluctuations in the noise is equal to the mean power P of the radiation.
Proposals have already been made for locking the modes of a laser emission by various different means, symbolized by the rectangle 4 in the FIG. 2 diagram of a laser source; with said means all relying on periodically modulating the loss or the gain to which the light is subjected on its back-and-forth passes in the resonant cavity between the mirrors 1 and 2, with the modulation being at the frequency df=c/2L as defined in equation (2). This periodically enhances preferential amplification of certain regions in the periodic noise i(t). Thus, a particular sequence of duration T.sub.O of pulses spaced apart in the period T=2L/c become progressively less noisy and increases to the detriment of regions which do not possess the optimum phase. The laser signal of intensity i(t) loses its random character and takes on the regular shape shown in FIG. 5. The frequency spectrum I(f) also takes on the regular shape shown in FIG. 6 with a sequence of regularly spaced-apart peaks at intervals df=c/2L, with each peak having a width 1/T.sub.O.
The laser emission regime shown in FIGS. 5 and 6 is generally designated by the terms "oscillator mode locking, phase locking, or synchronization". In this type of laser emission, the energy is concentrated into narrow pulses which are much more powerful than the mean power and which are capable of presenting a maximum power P.sub.max .apprxeq.P.(T/dt) (3) where T represents the period 2L/c in equation (1), and dt represents the reciprocal of the spectrum width .DELTA.f.
By way of example, an emission having a mean power P=1 watt as produced by a neodymium-doped YAG crystal (with .DELTA.f=10.sup.11 Hz) in a resonant cavity having a length of 1.5 m, contains pulses having peak power of about 1,000 watts.
By obtaining high instantaneous powers from much smaller mean power it is possible to enlarge the field of application of laser sources, and the use of a laser emission of the phase mode-locked type thus presents increasing practical advantages. However, the various known solutions for obtaining phase mode-locked laser emission are not fully satisfactory at present.
Thus, with pulsed type laser generators in which the amplifying medium is flash-pumped, so-called "passive" phase locking is achieved by introducing a medium in the cavity having the property that is transparency changes as a function of light intensity. The medium is a mixture of a dye and a solvent (chlorobenzene, dichloroethane, etc.) and is referred to as a "saturable absorbent". This liquid is contained in a cell and has very low transparency at low levels of illumination, while its transparency becomes very high when the incident intensity exceeds a characteristic value: the saturation intensity. A light pulse whose intensity exceeds this threshold is transmitted with negligible attenuation. The emission from mode-locked pulse lasers is in the form of a sequence of about 15 short (30 ps) pulses spaced by about 10 ns, and amplitude modulated by an envelope whose time profile is Gaussian. The most powerful pulse conveys energy of about 1 mJ.
The use of a mixture of dye and solvent for obtaining phase locking suffers from several drawbacks, and the main drawbacks are the following:
dye density is critical and not very stable over time;
the saturable absorbent solution suffers from wear and must be frequently renewed (about every 40 hours of utilization);
these absorbents exist only for a small number of emission wave lengths and are not suitable for all types of laser;
these absorbents do not impose any upper limit on the intensity of the laser pulses, thus giving rise to poor power stability and to risk of damage to the component parts of the laser; and
if the pumping repetition rate exceeds 1 Hz, it is necessary to provide means for circulating the dye.
For continuous laser generators, so-called "active" phase locking is generally used. For continuous pumping, the method of synchronizing the laser emission modes consists in periodically modulating the losses in the cavity at a frequency equal to c/2L of equation (2) above, by means of an acousto-optical modulator. Only the light signal which is in phase with the modulation can be amplified, such that the emerging laser beam is constituted by a continuous sequence of short light pulses, e.g. lasting about 100 picoseconds in YAG/Nd and argon lasers, which are spaced by 10 nanoseconds, for example. The energy of each of these pulses is about 10.sup.-8 J.
This type of phase locking the modes of a continuous laser generator also suffers from drawbacks. Each emitted laser pulse results from a large number of round trip passes through the laser cavity. The modulation frequency of the acoustooptical element (which generally lies in the range 80 MHz to 100 MHz) must always correspond exactly to the reciprocal of the time taken by one round trip pass through the oscillator. As a result the modulator must be extremely stable and its stability must be exact to within about 10 Hz. The microwave frequency sources used must therefore be crystal stabilized and the modulation frequency must also be periodically readjusted in order to keep track of variations in the length of the cavity due to temperature variations. Further, and in general, the acousto-optical technique and its modulation source are expensive.
Overall, the various devices for phase locking the modes of a laser do not satisfy all of the practical desiderata.
An article by A. Barthelemy, S. Maneuf, and C. Froehly entitled "Soliton propagation and self-confinement of laser beams by Kerr optical non-linearity" published in the journal "Optics Communications" vol. 55, No. 3, Sept. 1, 1985, pp. 201-206, describes a method of obtaining reproducible stable propagation of intense laser radiation in the presence of self-induced variations of the refractive index of a medium whose refractive index depends on intensity.
However, the utilization of the soliton effect has been considered, in practice, only for highly specific applications such as shortening picosecond pulses in monomode optical fibers, as described, for example, in the article by L. F. Mollenauer, R. H. Stolen, and J. P. Gordan entitled "Experimental observation of picosecond pulse narrowing and solitons in optical fibers" published in the journal "Physical Review Letters" vol. 45, No. 13, Sept. 29, 1980, pp. 1095-1098.
A device called a "soliton laser" is proposed in an article by L. F. Mollenauer and R. H. Stolen entitled "The soliton laser" published in the journal "Optics Letters" vol. 9, No. 1, Jan. 1984, pp. 13-15. According to this article, a color-center mode-locked laser including conventional acousto-optical means for additional active modulation is coupled with a second "soliton pulse" cavity for providing external regulation to the main laser cavity of the conventional basic laser generator. The external regulation loop comprises a single mode non-linear dispersive fiber in which a picosecond pulse propagates. In order to ensure that the fiber possesses dispersion of appropriate sign, it is necessary for the basic laser generator to be of the continuous type with a wavelength of more than 1.3 .mu.m.
Such a device suffers from all of the drawbacks mentioned above for mode-locked continuous laser generators and in particular from the need to use additional active modulation of the acousto-optical type, and it requires a second cavity to be implemented outside the cavity of the basic laser generator, together with the use of a non-linear dispersive fiber. The device is thus complex, bulky, difficult to adjust, and can only be used for a well-defined type of laser generator.